Auditory Temporal Processing in Developmental Dyslexics

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Abstract
Developmental dyslexia is usually considered to result from a high level cognitive/linguistic deficit. But it is possible that the phonological difficulties that most display may in part be due to impaired auditory perception. This study investigates the ability of adult dyslexics, screened for normal hearing sensitivity, to perform a non-speech auditory task. The test (FM rate difference limen) was designed to be analogous to visual motion sensitivity at which dyslexics are worse than controls. Subjects were identified as dyslexic by educational psychologists using standard criteria. Their persistent reading difficulties were confirmed using single real word and non-word reading tests. The 12 dyslexics not only made more errors than 12 matched controls in both tests but also took longer to complete the lists. When we ranked all 24 subjects on their word and non-word reading performance there was a strong correlation between them (r= 0.88; p< 0.001). There were no significant differences between dyslexics and controls in hearing 1 KHz pure tones; but the dyslexics were significantly worse at detecting changes in the rate of modulation of frequency modulated tones (their FM rate difference limen was larger; t = 3.04, p < 0.005); and their FM depth difference limen was also worse (t = 3.55, p < 0.005). The rank of all the subjects on these tests correlated with their reading performance (r = 0.5, p <0.05). Results support the hypothesis that dyslexics have impaired development of neuronal systems responsible for processing the timing of auditory frequency changes. As in the visual system this function may be mediated by magnocellular neurones.